The laminated glass manufactured by interposing an interlayer comprising a sheet made of a thermoplastic resin such as plasticized polyvinyl butyral between glass sheets and bonding them together into an integral unit is in broad use for glazing the windows of automobiles, aircraft, and buildings.
When such a laminated glass is subjected to an external impact, the glass may break up but the interlayer sandwiched between the component glass sheets will not readily be destroyed and even after breakage, the glass remains glued to the interlayer so that its fragments will not be scattered. Therefore, the bodies of men in the vehicle or building are protected against the injury by fragments of the broken glass.
Such a laminated glass is usually manufactured by interposing an interlayer between glass sheets, drawing the whole over a nip roll or placing it in a rubber bag and evacuating the bag to effect preliminary contact bonding with concurrent removal of the residual air entrapped between the glass and the interlayer under suction, and finally carrying out final contact bonding at elevated temperature and pressure in an autoclave.
The interlayer mentioned above is required to satisfy not only the basic performance requirements such as good clarity, bond ability, bullet resistance, weather resistance, etc. but also the requirement that it does not undergo blocking during storage, the requirement that it provides for good workability in the insertion thereof between glass sheets, and the requirement that it lends itself to efficient deaeration in preliminary contact bonding so that the formation of bubbles by entrapment of air may be precluded.
To satisfy the above requirements, it is common practice to provide both surfaces of an interlayer with many embossment patterns comprising fine convex portions and concave portions. As the geometry of such concave and convex portions, there are disclosed a variety of embossment geometries each containing a multiplicity of concave portions and the corresponding multiplicity of concave portions, and a variety of geometries each containing a multiplicity of ridges and the corresponding multiplicity of troughs.
The morphological parameters of an embossment design, such as coarseness, arrangement and relative size, have also been explored and Japanese Kokoku Publication Hei-1-32776 discloses “a thermoplastic resin interlayer comprising a flexible thermoplastic resin film or sheet having a fine concavo-convex (embossed) surface pattern for use as an interlayer for lamination characterized in that at least one side of which is provided with a multiplicity of discrete protruded portions integral with the film or sheet, with all the concave portions complementary to said protruded portions forming a continuum on the same level.”
However, when such an orderly embossment pattern is generally formed on both sides of the interlayer, the mutual interference of the diffracting surfaces gives rise to a streaks-like diffraction image known generally as the “moiré phenomenon”.
Furthermore, since the conventional embossment pattern is generally provided in a random fashion by using sand blasted roll, it hardly provides for sufficient deaeration.
The moiré phenomenon mentioned above is not only undesirable from appearance points of view but the attention-distracting change of the interference fringes causes an eye strain and motion sickness-like symptoms in the working personnel involved in interlayer cutting and laminating operations, thus leading to the problem of poor workability. Moreover, even in the case of an interlayer provided with an orderly embossment pattern only on one side, the operation involving the stacking of a plurality of interlayer sheets causes appearance of the moiré phenomenon, thus detracting from workability in a similar manner.
The moiré phenomenon is more liable to occur when the arrangement and pitch of the embossed pattern formed on the surface of an interlayer are more orderly, and in cases where the arrangement is such that the distance between at least two points of the convex portions of respective embossments is constant or where the arrangement of the embossment-pattern on both sides of the interlayer are identical, the moiré phenomenon occurs in most instances.
Therefore, such embossment patterns as a grid pattern, a stripe pattern, and a radiant pattern having a constant angular pitch may be mentioned as representative embossment patterns liable to give rise to the moiré phenomenon.
To overcome this disadvantage of the above moiré phenomenon and the associated deterioration of workability, Japanese Kokai Publication Hei-5-294679, for instance, discloses “a method which comprises providing the surface of an interlayer with a multiplicity of protruded portions in a controlled pattern and further with an embossment pattern of convex portions finer than these protruded portions in a random pattern.”
It is true that the above method contributes in a considerable measure to attenuation of the above moiré phenomenon but since the embossment pattern of finer convex portions is formed to extend not only to surfaces of the larger protruded portions but also surfaces not formed with the larger protruded portions, the pooling of air occurs in concave portions of the embossment between the finer convex portions so that the deaeration in preliminary contact bonding becomes insufficient as a disadvantage.
Further, Japanese Kohyo Publication Hei-9-508078 discloses an interlayer having embossment patterns each having an orderly array of troughs, the pattern on one side being displaced from that on the other side by not less than 25 degrees, more preferably by 90 degrees, to thereby obviate the moiré phenomenon.
It is known, in the above technology, that the linear designs displaced by 90 degrees for obviating the moiré phenomenon can be imparted by the heat transfer technique using a roll having engraved lines of 45 degrees. However, the larger the angle of engraved lines of the roll is, the less easy is the heat transfer to be effected. Generally speaking, a pattern of longitudinally parallel lines with respect to the flow of transfer can be most easily formed and a pattern of transverse lines requires transfer temperature control as well as a high transfer pressure.
Furthermore, in the above technology, unless the temperature at initiation of deaeration in preliminary contact bonding is critically controlled, a premature sealing of the marginal part of the glass-interlayer assembly (e.g. glass/interlayer/glass), i.e. premature marginal sealing, takes place, with the result that the deaeration of the central part of the assembly becomes still more inadequate.
As a measure to prevent the above premature marginal sealing, there is known the method which comprises controlling the temperature at initiation of deaeration according to the size of troughs to thereby prevent said premature sealing at the pressure bonding of the assembly or the method which comprises increasing the coarseness of embossment. However, there is the problem that in order to achieve a positive marginal seal of the laminate, the temperature for preliminary contact bonding must be considerably raised.
Furthermore, if the linear designs on both sides of the interlayer are made parallel from moldability considerations, the problem will arise that the handleability of the interlayer particularly in terms of self-adhesiveness is adversely affected, i.e. the self-adhesion of the interlayer is increased.
In fact, the above prior art interlayer has been fairly improved in the tendency toward blocking during storage, handling workability, and the efficiency of deaeration in preliminary contact bonding but in the production of a laminated glass having a large surface area or a laminated glass with a large radius of curvature or in carrying out deaeration under the stringent conditions imposed by circumstances calling for increased productivity of laminated glass, for instance, there is the problem that the deaeration and sealing effects are not so satisfactory as desired.
Thus, when deaeration is to be carried out under such stringent conditions, it is difficult, in particular, to establish a uniform seal between the sheet glass and interlayer all over the area and, hence, deaeration and sealing become insufficient, with the result that in the final contact bonding performed under heat and pressure in an autoclave, pressurized air infiltrates through the seal defect to form air bubbles between the glass and the interlayer, thus frustrating to produce a laminated glass of high transparency.
The problem of such a seal defect can be resolved to a certain extent by strictly controlling preliminary contact bonding conditions within a certain very narrow range but the compatible temperature range is so narrow that the incidence of rejects due to air bubble formation is increased.
Moreover, when a laminated glass is manufactured using an interlayer such that both the geometry of embosses and the level of depressions are uniform all over as described in the above disclosure, the variation in thickness of the very interlayer film and the pair thickness difference consisting of the difference in thickness or the difference in the radius of curvature of the glass to be laminated cannot be sufficiently absorbed.
In addition, in the case of the prior art interlayer, it is necessary to prepare a large number of embossing rolls having different designs corresponding to various processing needs of users and manufacture many kinds of interlayer films embossed to various three-dimensional patterns compatible with the respective users' processing conditions, this being inefficient from productivity points of view.
Furthermore, when the preliminary contact bonding process involving deaeration by draw deaeration is compared with the process involving deaeration by vacuum deaeration, there is a marked difference in the conditions of deaeration, viz. whereas deaeration is effected at an elevated pressure in the former process, it is effected at a negative pressure in the latter process, so that in establishments having only one kind of equipment, there are cases in which preliminary contact bonding cannot be carried out.
As mentioned hereinbefore, the preliminary contact bonding technology involving deaeration is generally classified into a draw deaeration method in which the glass-interlayer assembly is drawn over a rubber roll and a vacuum deaeration method in which the assembly is placed in a rubber bag and subjected to a negative pressure to bleed air from the margin of the glass-interlayer assembly.
In the deaeration method involving the use of a negative pressure, the process starts with placing the glass/interlayer/glass assembly in a sufficiently cooled (e.g. 20° C.) rubber bag and starting deaeration. The vacuum hold time is set to about 10 minutes and after the air is sufficiently removed from the whole glass/interlayer/glass assembly, the temperature is raised to heat the assembly to about 110° C. By this procedure, the interlayer and glass are bonded almost completely tight. Then, the assembly is cooled to the neighborhood of room temperature and the preliminary laminated glass thus obtained is taken out and transferred to the final contact bonding stage.
When the vacuum deaeration method is adopted in the preliminary contact bonding stage, which comprises the above cycle of heating and cooling, it is necessary for enhanced productivity to set the initial temperature within the rubber bag at a high level and set the ultimate temperature at a low level.
However, when the initial temperature within the rubber bag is set high, the marginal part of the assembly is the first to succumb to the pressure of contact bonding so that the air in the central part is prevented from escaping efficiently but remains entrapped. If the deaeration is sufficient in the preliminary contact bonding stage, any residual air, which is small in amount, is allowed to dissolve in the interlayer in the final contact bonding stage (e.g. 130° C.×1.3 MPa×1 hr), with the result that a transparent laminated glass can be obtained. However, if the residual amount of air is large, the air will not be completely dissolved in the final contact bonding stage so that air bubbles appear in the product laminated glass. On the other hand, if the ultimate temperature is set too low, an incomplete seal occurs locally in the marginal region and as the pressurized air finds its way into such localities in the final contact bonding stage, air bubbles are produced in the product laminate.
Another factor contributory to the above phenomenon is that, in a laminated glass of the glass/interlayer/glass construction, there occur areas where one of the glass sheets is urged toward the other glass sheet and areas where one of the glass sheets is urged away from the other glass sheet depending on the accuracy of glass bending and the way in which the gravity of glass acts.
The geometry of embossed surface irregularities proposed so far includes random geometries (a hill and a valley are alternating) and orderly geometries comprising quadrangular pyramids or triangular pyramids. In addition, as applicable to the vacuum deaeration method, Japanese Kohyo Publication Hei-9-508078 teaches that providing a route for escape of air by means of troughs is effective in preventing the premature sealing in the course of deaeration.
This method, however, has the disadvantage that while the initial temperature within the rubber bag can be set high, the ultimate temperature must also be set high and if the ultimate temperature is set low, the infiltration of air will occur in the final contact bonding stage to cause air bubbles. Thus, in the case of the conventional random embossments, the heating may be carried out simply from an initial temperature of 20° C. to an ultimate temperature of 85° C. In the method referred to above, however, the formation of air bubbles cannot be avoided unless the heating is performed from an initial temperature of 35° C. to an ultimate temperature of 95° C. so that even if the depth (height), width, and pitch of troughs or ridges are optimized, the embossments must be collapsed to a certain volume. Consequently, the initial temperature and the ultimate temperature must be shifted upward almost in parallel, with the result that the effect of increasing the productivity of preliminary contact bonding, which is a deaeration process, is small.